1. Field of the Invention
The present invention relates to silver halide photographic emulsions which are spectrally sensitized, particularly, those which are fit to produce a red-sensitive emulsion layer of a multilayer color light sensitive material.
2. Description of the Prior Art
Spectral sensitization is an extremely important technique which has been used to not only impart a more appropriate sensitization distribution ranging from the intrinsic sensitization region to the longer wavelength region corresponding to green light and red light, but also to increase the total sensitivity (to white light). Particularly, since multilayer color photosensitive materials require respective photographic emulsion layers sensitive to not only blue light but also to green light and to red light, spectral sensitization is indispensable for the production of a green-sensitive emulsion layer and a red-sensitive emulsion layer.
Furthermore, it is required to impart a more suitable color reproducibility to color light sensitive materials. Therefore, the improvement in the characteristics of spectral sensitization in a green-sensitive layer and a red-sensitive layer becomes, from the standpoint of obtaining excellent color reproducibility, significant. In particular, it is important at what wavelength to make the red-sensitive layer selectively exhibit a sensitization maximum because the wavelength of the sensitization maximum of the red-sensitive layer delicately affects the reproducibility of skin color, which is a significant factor in color photographs, and also influences variations of color balance caused by changing the light source, as disclosed in Japanese Patent Publication 6207/74. For example, it is disclosed in Japanese Patent Publication 6207/74 that the selection of about 605 nm (.+-.5 nm) as the wavelength of the sensitization maximum of the red-sensitive layer in a multilayer color light sensitive material is desirable to maintain the color balance under the best conditions when three kinds of illumination means are used, i.e., day light, a tungsten lamp and a fluorescent lamp. However, it is not preferred, for the purpose of producing highly sensitive color light sensitive materials employed for picture taking, to choose the above described wavelengths as the sensitization maximum of the red-sensitive emulsion layer because such a sensitization maximum will not impart high sensitivity to the color light sensitive material, though it can lead to the smallest variations in color balance with the kind of light sources used, and, therefore, it is necessary to render the grain size of silver halide in the photographic emulsion large to increase sensitivity, which results in a sacrifice of graininess and sharpness.
It was found that the conditions where the red-sensitive layer have a sensitization maximum within the wavelength region ranging from 625 nm to 645 nm and a spectral sensitivity corresponding to at least 40% of the maximum spectral sensitivity in the wavelength region ranging from 580 nm to 600 nm were the most advantageous conditions to obtain highly sensitive color light sensitive materials which possess a satisfactory high color reproducibility without harming graininess and sharpness. There are known many sensitizing dyes which show a sensitization maximum in the aforesaid wavelength region, but all of these known sensitizing dyes cannot impart a sufficiently high spectral sensitivity in the wavelength region ranging from 580 nm to 600 nm to photographic emulsions.
Moreover, it is required to take measures so that the spectral sensitization action inherent to a sensitizing dye is not inhibited by couplers which coexist therewith, since modern multilayer color light sensitive materials contain color image-forming couplers in the respective photographic emulsion layers. Furthermore, coloration caused by residual dye must not appear after photographic processings, since the hue of the color photograph obtained is remarkably impaired by such coloration.